The Zuider Zee (Dutch: 'southern sea') was a shallow inlet of the North Sea which divided the north-eastern and north-western parts of the Netherlands. The sea was of relatively recent formation. Until the 13th century the northern coast of what is now the Netherlands was a solid land mass; a large lake, named Flevo by the Romans, occupied the centre of the country. However, rising seal levels were eroding the land barrier, which was opened by catastrophic storms in 1282. Subsequent to this event, the name Zuider Zee came into use, and the changed geography of the Netherlands also permitted the village of Amsterdam to grow as a port with access to the North Sea via the new sea.
In 1932, with the completion of the Afsluitdijk embankment across the mouth of the inlet, the Zuider Zee ceased to exist and became instead the Ijsselmeer lake. In succeeding decades, a number of large polders were created from the bed of the lake.